Japanese encephalitis (JE) is caused by Japanese encephalitis virus (JEV) transmitted through the bite of infected Culex mosquitoes. JE is endemic throughout most of Asia and parts of the western pacific regions (1). Globally, 100,000 clinical cases were estimated to occur annually and 25,000 were estimated to die (2). Additionally, outbreaks of JE have occurred in several previously non-endemic areas such as Australia in recent decades (3). In Japan, JE used to be endemic, and more than 1,000 JE patients had been reported before the 1960s (4). However, the introduction of the JE vaccine in 1954 and its widespread childhood vaccination from 1967 dramatically changed the disease burden, and only 10 or fewer cases have been reported annually in Japan 5, 6. As of 2022, only one case each had been reported in Chiba and Ibaraki prefectures, respectively in the past 10 years, and both cases occurred in areas famous for pig farming far from Narita city 7, 8.

Diagnosis of JE is challenging because its symptoms are non-specific and commercially available diagnostic tests in Japan are non-specific. The most commonly recognized presentation is acute encephalitis; however, mild symptoms such as aseptic meningitis or undifferentiated febrile illnesses can occur. Furthermore, asymptomatic infection consists of most cases (9). As for the diagnostic tests, specific diagnostic tests such as a JE virus-specific IgM-captured ELISA on CSF or serum or neutralization tests are only available in specific reference laboratories (10). These situations suggest that JE may be underdiagnosed in Japan.

Here, we report three domestically infected JE cases over the three consecutive years from a tertiary care hospital in Narita, Chiba, where JE had not been reported in recent years.